FIG. 1a illustrates a conventional power device having a wire bonded lead frame. FIG. 1b illustrates a cross-sectional view along line A-A in FIG. 1a. A set of leads (1, 2, 3) is interconnected to a power chip 7 via wires that are soldered to each lead in the set of leads (1, 2, 3) and a corresponding pad (4, 5, 6) provided on the power chip 7. The power chip 7 is provided on a substrate 8 and the resulting structure is encased in a plastic encapsulation 9. The wire bonding is performed as a separate operation after die attaching the power chip 7 to the substrate 8. The leads (1, 2, 3) may each correspond to a gate, source, and drain of the power chip 7, where the source and drain are high current leads and the gate is a low-current lead for a control signal.
In this conventional power device, the wiring requires the use of special bonding techniques, such as ultrasound wire bonding. The required bonding material and wires add resistance to the flow of current. Accordingly, even if the power chip 7 itself is capable of operating with a particular amount of current, the bonding material and wires limit the maximum current of the device to an amount that is less than the amount which the power chip 7 is capable of withstanding.
FIG. 2a illustrates a conventional power device having clip joints. FIG. 2b illustrates a cross-sectional view along line A-A in FIG. 2a. A set of pins (30, 31, 32) is connected to a power transistor 39 via clips (21, 22, 23). Each clip (21, 22, 23) is shaped as a single lead. The power transistor 39 is provided on a metal can package 40. One end of each of the clips (21, 22, 23) is bonded, via a solder joint (24, 25, 26), to a pad (36, 37, 38) coupled to the power transistor 39. The other end of each of the clips (21, 22, 23) includes a hole (33, 34, 35) through which one of the pins (30, 31, 32) extends through and is soldered to an inner surface thereof. The clips (21, 22, 23) may each correspond to one of the base, emitter, and collector of the power transistor 39. Each of the pins (30, 31, 32) also extends through a hole (27, 28, 29) in the metal can package 40.
In this conventional power device, the solder joints (24, 25, 26) are susceptible to mechanical stress between the clips (21, 22, 23) and the power transistor 39, which may result in deterioration from thermal fatigue. The power transistor 39 also has a limited current capacity due to structural elements such as the solder used to couple the pins (30, 31, 32) to the clips (21, 22, 23).
Accordingly, an improved apparatus is desired that solves some of the problems and reduces some of the drawbacks discussed above.